Free-space optical communications systems are systems in which beams of modulated light are transmitted from transmitters to receivers through free space (air). Such systems can be used to provide telecommunications services in areas in which it is difficult or expensive to provide hard-wired network connections using twisted pair wiring, coaxial cable, or optical fiber. It may be desired to use wavelength division multiplexing in a free-space optical communications system to increase the information carrying capacity of the system. By using multiple wavelengths of light in such systems, wavelength division multiplexing allows multiple channels of information to be carried between transmitters and receivers. At the receivers, the multiple channels are demultiplexed by wavelength filtering.
In wavelength division multiplexing systems in which optical signals are transmitted between network nodes solely on single mode optical fiber, a single mode fiber-based demultiplexer may be used to perform wavelength filtering. However, fiber-based demultiplexers can be difficult to use. For example, it is extremely difficult to couple free space transmissions into single mode fiber-based demultiplexers.
It is therefore an object of the present invention to provide improved demultiplexers and receivers for use in free-space wavelength division multiplexing optical communications systems.